In the combustion device such as a combustor of a gas turbine, it is required to achieve both of diversification of fuel allowing to cope with various kinds of fuel and improvement of environmental performance.
With respect to diversification of fuel, in a part of the gas turbines, a so-called dual fuel combustor is employed which uses a liquid fuel such as light oil and heavy oil A in addition to fuel gas represented by natural gas according to the supply condition of the fuel. In this combustor, because both of a gas fuel and a liquid fuel can be used, operation can be continued even when the supply condition of the fuel changes.
With respect to improvement of the environmental performance of a gas turbine, reduction is required of nitrogen oxide (NOx), carbon monoxide (CO), and particulate matters discharged mainly by combustion. With respect to reduction of nitrogen oxide, development is underway mainly for reduction of thermal NOx generated by oxidation of nitrogen in the air at a high temperature. For reduction of thermal NOx, application of the lean premixed combustion method is effective in which combustion is performed after excessive air is mixed to the fuel beforehand. According to this combustion method, the ratio of the fuel and air in the combustion space becomes constant, the air volume is so much that localized high temperature field is not formed, and then formation of thermal NOx can be suppressed. On the other hand, because the rate of the air to the fuel is high, stable combustion range is limited. Also, when the combustion temperature is low, CO and particulate matters are liable to be generated as unburnt carbon.
Even when a liquid fuel is used for a gas turbine combustor, application of the lean premixed combustion method is preferable for reduction of nitrogen oxide. In the case of the liquid fuel, for the purpose of lean premixed combustion, it is necessary that the fuel component of the liquid state is vaporized beforehand and is mixed with air before combustion. At this time, when the liquid fuel is atomized, the surface area per weight of the liquid fuel increases and vaporization is performed easily. Further, also in the case the liquid fuel is combusted without vaporization by atomizing the liquid fuel, combustion reaction becomes quick because the surface area per weight increases. Therefore, unburnt carbon is hardly generated, and the amount of CO and particulate matters generated from the combustion device can be reduced. Thus, in combustion of a liquid fuel, atomization is one of the important factors for improvement of the environmental performance.
As one of the atomizers to atomize a liquid fuel, there is a twin-fluid atomization method in which a liquid fuel (spray fluid) and gas such as air and steam (spray medium for atomization) are supplied, mixed with each other, and thereby atomized. In general, the twin-fluid atomization method has higher atomization performance in spraying of a large volume compared with a pressure atomization method of atomizing a spray fluid without using a spray medium. Also in the twin-fluid atomization method, even when the supply amount of the liquid fuel may change, variation in atomizing performance is small. Therefore, the twin-fluid atomization method is generally used for a combustion device in which the combustion load changes.
In an atomizer of the twin-fluid atomization method (hereinafter referred to simply as “atomizer”), it is required to improve the atomization performance, to reduce the used amount and the pressure of the spray medium, and to reduce the energy usage required for spraying. Therefore, the mixing method of the spray medium has been studied.
JP-A 62-186112 discloses an example of the atomizer of an inside mixing method in which the spray fluid and the spray medium are mixed in a space (mixing chamber) on the upstream side, and the fluid after mixing (hereinafter referred to as “fluid mixture”) is ejected from a plurality of outlet holes. In the atomizer described in JP-A 62-186112, mixing advances because the flow directions of the spray fluid and the spray medium change in the mixing chamber, and then the spray fluid is atomized. The atomization of the spray fluid advances because the fluid mixture is ejected from the outlet holes at a high speed, generating speed difference against the ambient gas and applying a shear force to the spray fluid in the fluid mixture.
JP-A 9-239299 discloses an atomizer in which the fluid mixtures of the spray fluid and the spray medium are made to flow to oppose each other and to collide with each other in the vicinity of an outlet hole, and thereby atomization of the spray fluid is promoted. The atomizer as described in JP-A 9-239299, in which the fluid mixture is ejected in a fan-shape from the outlet hole, is also called a fan spray type atomizer because of the spraying shape. In the fan spray type atomizer, in addition to an effect that the spray fluid is mixed with the spray medium and is atomized, the fluid mixture is ejected in a fan-shape from the outlet hole, therefore the interface of the fluid mixture and the ambient gas is so much that a shear force is applied to the spray fluid in the fluid mixture by the speed difference against the ambient gas, and atomization of the spray fluid advances.
The twin-fluid atomizer disclosed in JP-A 62-186112 mixes the spray fluid and the spray medium in the mixing chamber, and ejects the fluid mixture from a plurality of outlet holes. Atomization is executed mainly by mixing by stirring (stirring mixing) in the mixing chamber and by a shear force caused by the speed difference against the ambient gas generated when the fluid mixture is ejected from the outlet holes at a high speed. However, the spray fluid and the spray medium are liable to be separated because of the difference in density. Particularly, in the mixing chamber where the volume is large and the flow speed of the fluid mixture drops, the spray fluid and the spray medium are separated at a portion with low flow speed, and local variance of the mixing ratio of the spray fluid and the spray medium is liable to become large. In this case, there is a problem that stirring mixing in the mixing chamber does not advance, and atomization does not advance. Further, there is also a problem that, when the spray fluid and the spray medium are made to collide on the wall surface on the downstream side of the mixing chamber to promote stirring mixing, the solid portion (coarse particles) in the spray fluid collides on the wall surface, thereby the wall surface is worn, and the frequency of exchange of the atomizer increases.
The twin-fluid atomizer disclosed in JP-A 9-239299 mixes the spray fluid and the spray medium in a plurality of mixing sections. However, there is a problem that, because the fluid mixture flows at a high speed, the mixing time inside the flow passage is short in general, and therefore mixing does not advance sufficiently. Further, there is also a problem that, because the flow passage of the spray fluid connected from the upstream side and the flow passage of the spray medium cross with each other at the mixing section, the solid portion in the spray fluid collides on the wall surface of the mixing chamber, and thereby the wall surface is liable to be worn.
The object of the present invention is to provide an atomizer capable of promoting atomization of the spray fluid and capable of suppressing the wear caused by the solid portion in the spray fluid, and to provide a combustion device including the atomizer.